The present invention relates to a fail-safe track circuit receiver comprising at least one synchronous detector, at least one low-pass filter, and following threshold switches.
German Offenlegungsschrift (DT-0S) No. 2,217,133 describes a receiver for evaluating track voltages in track circuits for track-clear signalling in railway systems which contains several synchronous detectors followed by low-pass filters whose output voltages actuate track relays via switching stages.
The reference voltages necessary for phase demodulation may have predetermined phase differences in order to obtain a "clear" indication of the track section in a given range of the phase position of the track voltage to be evaluated. This circuit permits fail-safe track-clear signalling as long as the phase of the received ac voltage is fixed relative to that of the ac reference voltage. Since the phase of the ac reference voltage is preset in this case, it cannot follow the received ac voltage, whose phase varies with weather conditions, and this results in phase shifts between the two voltages which lead to a decrease in the level of the dc voltage signals at the outputs of the low-pass filters. This decrease is possible mainly in track circuits of great length and in insulated-rail-joint-free track circuits, and results in false indications of track conditions and, thus, in obstructions to rail traffic.